CN117209754A - DOPO-based halogen-free intrinsic flame retardant nylon 66 and preparation method thereof - Google Patents

Abstract

The invention provides a DOPO-based halogen-free intrinsic flame retardant nylon 66, which is produced by copolymerizing a dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant and PA66, in which the dicarboxy-terminated DOPO-based halogen-free reactive flame retardant It is a flame retardant molecule with the structure shown in Formula I: where ‑R is selected from where a is an integer of 1-7; the flame retardant molecule with the structure shown in formula I is first reacted with hexamethylenediamine in a ratio close to 1:1 to prepare a flame retardant salt with one carboxyl group and one terminal amino group. The flame retardant salt is then mixed and copolymerized with the nylon 66 salt at a molar ratio of (10-1): (90-99), so that the flame retardant molecule with the structure shown in Formula I can be firmly connected to the nylon 66 molecular chain, improving nylon 66 has flame retardant properties and will not precipitate due to poor compatibility of flame retardants, ensuring the stable mechanical properties of nylon 66.

Description

A DOPO-based halogen-free intrinsic flame-retardant nylon 66 and a preparation method thereof

Technical Field

The invention belongs to the technical field of flame-retardant polyamide materials, and specifically relates to a DOPO-based halogen-free intrinsic flame-retardant nylon 66 and a preparation method thereof.

Background Art

Polyamide (PA), also known as nylon, is a thermoplastic resin containing amide bonds (-CO-NH-) in the molecular chain. Polyhexamethylene adipamide resin (PA66) is the most widely used polyamide material. It has good mechanical properties, heat resistance, chemical resistance and wear resistance, and is easy to modify and process. It is widely used in the fields of automobiles, machinery, electronics, chemicals, construction, etc., and has become one of the engineering plastics with the largest output and the widest application range in the world. However, PA66 itself has a low flame retardant level, an oxygen index of 24%, and a low vertical combustion level. It may cause fire during use, which limits its application. Therefore, improving the flame retardant properties of PA66 through modification has always been a very important research topic.

At present, two methods are commonly used to modify PA66 for flame retardancy: 1) Additive flame retardant, that is, adding flame retardant to the matrix, dispersing flame retardant in the matrix by extrusion blending, and improving the flame retardant performance of the substrate. This method is easy to process and requires little equipment investment, and is currently the most widely used technology. However, to achieve a higher flame retardant level, the amount of flame retardant added is often large, which can easily reduce the mechanical properties of the material. 2) Reactive flame retardant, that is, the flame retardant is combined with the macromolecular chain as a reaction unit through chemical reaction, making it a flame retardant component in the structural unit. It has a high flame retardant efficiency, overcomes the shortcomings of additive flame retardant migration or volatilization from the polymer surface, and can maintain the original physical, chemical and mechanical properties of the polymer, but there are huge challenges in technology, equipment and cost.

Traditional flame retardants are classified according to the type of flame retardant elements, including halogen flame retardants, phosphorus flame retardants, nitrogen flame retardants, silicon flame retardants, inorganic nanofiller flame retardants, and compound flame retardants of multiple flame retardants. Since halogen flame retardants release toxic gases such as hydrogen halides during combustion, as people's environmental awareness increases, the use of halogen flame retardants has gradually decreased; among halogen-free flame retardants, phosphorus and nitrogen flame retardants are the most widely used. Phosphorus flame retardants mainly play a role in the condensed phase. When combustion occurs, phosphorus flame retardants will be thermally decomposed to generate free radicals, which can quench the active free radicals required for the combustion reaction chain transfer, thereby achieving the purpose of flame retardancy. In addition, phosphorus flame retardants often produce phosphoric acid, which will cause the polymer surface to form a dense non-combustible carbon layer, covering the surface of the polymer material to form a protective film, preventing the exchange of substances and energy inside and outside the film, so as to achieve flame retardancy.

9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) is a common reactive phosphorus-based flame retardant. The C-P bond in its molecule forms a phosphaphenanthrene structure together with biphenyl. It has high thermal stability and a decomposition temperature of 180°C, which can meet the processing requirements of most plastics. However, for PA66, its processing temperature reaches 260-280°C, so it is very meaningful to design a DOPO flame retardant with a high decomposition temperature.

The Chinese invention patent with application number CN 202211086418.X discloses a series of phosphorus-containing reactive flame retardants, which are applied to polyamides such as PA66, PA6, and PA46 to prepare a series of flame-retardant copolymer nylon materials. However, due to the complex structure of the flame retardant, the difficulty in synthesis and the high cost, it is difficult to apply on a large scale.

The Chinese invention patent with application number CN98807508.3 discloses a reactive phosphorus flame retardant CEPPA for polyamide, which is obtained by melt copolymerization with PA66 salt to obtain flame-retardant copolyamide. However, due to the low decomposition temperature of the flame retardant (275°C), it cannot meet the traditional PA66 polymerization process requirements, so caprolactam is added for copolymerization to reduce the polymerization temperature and ensure its processing performance. However, this practice also makes PA66 lose its original high-temperature performance advantage.

In view of this, the present invention is proposed.

Summary of the invention

The purpose of the present invention is to provide a DOPO-based halogen-free intrinsic flame-retardant nylon 66 in view of the above-mentioned problems in the prior art. By introducing DOPO groups into flame retardant molecules, the nylon 66 has excellent flame retardant properties. At the same time, the flame retardant molecules are smoothly introduced into nylon 66 by utilizing the dicarboxylic acid introduced into the flame retardant molecules, thereby effectively avoiding the precipitation of the flame retardant and thus avoiding the influence on the mechanical properties of nylon 66 itself.

The present invention also provides a method for preparing DOPO-based halogen-free intrinsic flame-retardant nylon 66. The preparation method is simple and easy to implement, reduces production costs, and is conducive to industrial application and promotion.

In order to achieve the above-mentioned object, the first aspect of the present invention provides a DOPO-based halogen-free intrinsic flame-retardant nylon 66, which is prepared by copolymerizing a dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant with PA66, wherein the structure of the dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant is as shown in Formula I:

Wherein, -R is selected from one of C ₁ -C ₈ alkyl, phenol, furyl, aryl, heteroaryl, alkyl/alkoxy substituted aryl, and alkyl/alkoxy substituted phenol;

Preferably, -R is selected from one of phenol, furanyl, aryl, heteroaryl, alkyl/alkoxy substituted aryl, and alkyl/alkoxy substituted phenol;

More preferably, -R is selected from wherein X is a C ₁ -C ₁₀ alkyl group;

More preferably, X is methyl.

In the above scheme, the flame retardant molecules can be introduced into the nylon 66 molecules through the carboxyl group, so there will be no problem of poor compatibility between the flame retardant and nylon 66, ensuring that nylon 66 has excellent flame retardant effect, and no flame retardant precipitation will occur, which will not affect the physical properties of nylon 66.

Furthermore, the flame retardant molecule is prepared by an addition reaction between DOPO and an imine bond on a substance having a structure as shown in Formula II;

In the above scheme, the substance shown in Formula II has two carboxyl groups, which can undergo addition reaction with the imine bond in nylon 66 salt, thereby smoothly introducing the flame retardant molecules into the molecular chain of nylon 66, thereby realizing the intrinsic flame retardancy of nylon 66; the introduction of electron-withdrawing groups on the benzene ring can improve the thermal stability of the flame retardant, which is more conducive to its copolymerization, and can also promote carbonization and improve the flame retardancy of the material.

Furthermore, the preparation method of the dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant of the structure shown in Formula I is as follows:

S1. Under an inert atmosphere, a 5-aminoisophthalic acid solution and an aldehyde compound solution having a structure as shown in Formula III are mixed, and reacted at a preset temperature to obtain a mixed solution of a substance having a structure as shown in Formula II;

S2. Add DOPO to the mixed solution obtained in step S1 to react and obtain a DOPO-based halogen-free reactive flame retardant having a terminal dicarboxyl group and a structure as shown in formula I.

Furthermore, step S1 is specifically as follows:

Under an inert atmosphere, a 5-aminoisophthalic acid solution and an aldehyde compound solution having a structure as shown in Formula III are mixed, the temperature is raised to a preset first temperature and continuously stirred, and then the temperature is lowered to a preset second temperature and continuously stirred.

Preferably, in step S1, the preset first temperature is 5-20°C higher than the boiling point of the solvent; and the preset second temperature is 10-50°C lower than the boiling point of the solvent.

Preferably, in step S2, the reaction temperature is 5-20° C. higher than the boiling point of the solvent.

The above scheme controls the reaction process to first raise the temperature to a temperature higher than the boiling point of the solvent and then lower the temperature to a temperature lower than the boiling point of the solvent. While ensuring the reaction rate, the occurrence of side reactions is reduced by lowering the temperature in the later stage, thereby improving the yield. The limitation on the reaction time ensures that the reaction can proceed fully, further improving the yield. In step S2, the reaction temperature is also controlled to be higher than the boiling point of the solvent, so that the dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant can be synthesized with higher efficiency, thereby improving the yield of the flame retardant to a certain extent.

The present invention also provides a method for preparing the above-mentioned DOPO-based halogen-free intrinsic flame-retardant nylon 66, comprising the following steps:

S1. Mixing a DOPO-based halogen-free reactive flame retardant having a dicarboxyl terminal group and a structure as shown in Formula I with hexamethylenediamine to prepare a flame retardant salt;

S2. The flame retardant salt is mixed with nylon 66 salt to carry out polymerization reaction to obtain DOPO-based halogen-free intrinsic flame retardant nylon 66.

Furthermore, in step S1, the dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant is mixed with hexamethylenediamine, and then a solvent is added. The mixture is heated and stirred for a preset time to form a mixed solution, and the mixed solution is post-treated to obtain a flame retardant salt.

Preferably, the molar ratio of the dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant to hexamethylenediamine in the mixed solution is 1:1.2.

The preparation process is as follows:

The above scheme prepares the flame retardant salt by mixing a DOPO-based halogen-free reactive flame retardant having a dicarboxyl terminal group with hexamethylenediamine, and by controlling the molar ratio of the two, one of the carboxyl groups in the flame retardant reacts with the terminal amino group of hexamethylenediamine to prepare a flame retardant salt having one carboxyl group and one terminal amino group, and its terminal group is the same as that of nylon 66, so it can more smoothly enter the molecule of nylon 66.

Furthermore, the solvent of the mixed solution is DMSO; the mixing temperature is 30-80° C.; the preset time is 0.5-2 h; and the stirring speed is 50-100 rpm.

Furthermore, the post-treatment is as follows: the mixed solution is cooled to room temperature and filtered under reduced pressure, and the flame retardant salt is obtained after washing and drying.

Furthermore, the washing and drying include: washing with anhydrous ethanol and drying at 60° C. for 24 hours, then washing with deionized water and drying at 95° C. for 24 hours.

In order to ensure the washing and drying effects, the specific operations of washing and drying are as follows: wash three times with anhydrous ethanol, put into a vacuum oven, and dry at 60°C for 24 hours; then wash three times with deionized water, put into a vacuum oven after washing, and dry at 95°C for 24 hours.

Furthermore, in step S2, the molar ratio of nylon 66 salt to flame retardant salt is (90-99):(10-1).

Preferably, the molar ratio of nylon 66 salt to flame retardant salt is (94-96):(6-4). This range has a balance between flame retardancy and mechanical properties and is a better feed ratio.

The above-mentioned ratio range is a more preferred technical solution obtained by technicians based on analysis of a large number of studies. Since the dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant enters the molecular chain of nylon 66, it inevitably affects the performance of nylon 66. By controlling the molar ratio of nylon 66 salt to flame retardant salt within the above-mentioned range, it is possible to significantly improve the flame retardant properties of nylon 66 while minimizing the influence of the introduction of flame retardants on the mechanical properties of nylon 66, so that the comprehensive performance of the obtained intrinsic flame retardant nylon 66 is excellent.

Furthermore, the polymerization reaction process is as follows: the flame retardant salt and the nylon 66 salt are mixed, the temperature is raised to a preset third temperature for reaction, and then the pressure in the reactor is reduced to below 100 Pa, and the temperature is raised to a preset fourth temperature to continue the reaction. After the reaction is completed, heating and stirring are stopped to obtain DOPO-based halogen-free intrinsic flame retardant nylon 66; wherein the preset third temperature is higher than the preset fourth temperature.

In the above scheme, by limiting the fourth temperature to be higher than the third temperature, the reaction initially carried out at the third temperature can be carried out at a higher speed as the temperature increases and the pressure decreases, thereby obtaining a product with a higher molecular weight, which is beneficial to the improvement of the mechanical properties of nylon 66; and the flame retardant salt used in the preparation method of the present invention has a structure as shown in Formula I, and has a higher thermal decomposition temperature than conventional flame retardants, thereby being able to meet the requirements of high-temperature polymerization of nylon 66, and further contributing to the full play of the excellent mechanical properties of nylon 66.

Preferably, the preset third temperature is 240-260°C;

Preferably, the reaction time at the preset third temperature is 1-2 hours;

Preferably, the preset fourth temperature is 250-270°C;

Preferably, the reaction time at the preset fourth temperature is 10-30 minutes.

The above-defined temperature range fully takes into account the reaction temperature requirements of the flame retardant salt and the nylon 66 salt in different ratios, thereby ensuring that DOPO-based halogen-free intrinsic flame retardant nylon 66 with intrinsic flame retardant properties can be successfully prepared in different ratios.

The beneficial effects of the present invention are:

1. By introducing a flame retardant molecule having a structure as shown in Formula I into nylon 66, the flame retardant molecule can enter the nylon 66 molecule, thereby achieving the intrinsic flame retardancy of nylon 66; the flame retardant molecule having the structure as shown in Formula I has a DOPO group, the DOPO group is connected to the imine group through a C=N bond, an N atom is introduced into the flame retardant molecule, and the DOPO molecule has a C-P bond, so that nylon 66 exhibits excellent intrinsic flame retardancy.

2. The two terminal groups of nylon 66 salt are carboxyl and amino respectively. The flame retardant molecule with the structure shown in Formula I first reacts with hexamethylenediamine, and a carboxyl group of the flame retardant molecule forms an amide bond with the terminal amino group of hexamethylenediamine. The obtained flame retardant salt also has the same terminal carboxyl group and terminal amino group as the nylon 66 salt, thereby allowing the flame retardant molecule to be more smoothly introduced into the nylon 66 molecule without causing the flame retardant to precipitate due to poor compatibility.

3. The introduction of flame retardant molecules will inevitably lead to changes in the molecular chain structure, which will in turn affect the excellent mechanical properties of nylon 66 itself. In order to improve the flame retardant properties of nylon 66 while avoiding a significant decrease in mechanical properties, the molar ratio of nylon 66 salt to flame retardant salt during the preparation process is controlled to be (90-99):(10-1), ensuring that nylon 66 has excellent flame retardant properties while maintaining mechanical properties, and the overall performance is significantly improved.

4. Compared with the blending modification method using traditional DOPO-based flame retardants, this technology has better flame retardant effect and better mechanical properties when the same flame retardant addition amount is used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a ¹ H-NMR graph of DOPO-based halogen-free intrinsic flame-retardant PA66 prepared according to the method described in Example 1 of the present invention.

FIG. 2 is a DSC test graph of DOPO-based halogen-free intrinsic flame-retardant PA66 and conventional PA66 prepared according to the methods described in Example 1 and Example 2 of the present invention.

FIG. 3 is a TGA test graph of DOPO-based halogen-free intrinsic flame-retardant PA66 and conventional PA66 prepared according to the methods described in Example 1 and Example 2 of the present invention.

FIG. 4 is a TGA test graph of the dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant and DOPO prepared by the method described in Example 1 of the present invention.

In the figure:

BADO represents a DOPO-based halogen-free reactive flame retardant having a terminal dicarboxyl group and a structure as shown in Formula I, obtained by the method of the present invention;

PA66-co-BADO _n represents a nylon 66 copolymer made from a dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant as a raw material;

n represents the mole fraction of BADO component in nylon 66 copolymer.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Those skilled in the art will appreciate that the following embodiments are only used to explain the technical principles of the present invention and are not intended to limit the protection scope of the present invention.

It should be noted that in the experimental example section of the specific implementation method, the performance test method for the DOPO-based halogen-free intrinsic flame-retardant nylon 66 prepared by the preparation method of the present invention is as follows:

The limiting oxygen index (LOI) test was conducted on a Fire Testing Technology apparatus (FTT, West Sussex, UK) according to ASTM D 2863. The sample size was 80.0×10.0×4.0 mm ³ .

The UL-94 vertical burning test was conducted on a CZF-5 instrument (Jiangning Analytical Instruments Co., Ltd., Jiangning, China) according to ASTM D 3801. The specimen size was 80.0×13.0×3.2 mm ³ .

The tensile properties were tested using a tensile testing machine (Instron 3365). The test samples with a thickness of about 0.5-0.8 mm were cut into dumbbell shapes. The test conditions for all samples were as follows: strain rate of 100 mm/min, spacing of 4 mm, testing at room temperature, relative humidity of 60±3%. Each sample was tested five times, and the average value was taken as the final test result of the sample.

Impact performance was tested using an impact tester (XJC-25D). Izod notch impact form, test temperature 23°C), relative humidity 60±3%, according to standard GB1043-2008.

The melt index is measured on a melt flow rate meter according to ISO 1133. The sample mass is 6 g and the test temperature is 230°C.

The present invention provides a DOPO-based halogen-free intrinsic flame-retardant nylon 66, which is prepared by copolymerizing a dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant with nylon 66, wherein the structure of the dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant is as shown in Formula I:

Wherein, -R is selected from one of C ₁ -C ₈ alkyl, phenol, furyl, aryl, heteroaryl, alkyl/alkoxy substituted aryl, and alkyl/alkoxy substituted phenol;

Preferably, -R is selected from one of phenol, furanyl, aryl, heteroaryl, alkyl/alkoxy substituted aryl, and alkyl/alkoxy substituted phenol;

More preferably, -R is selected from wherein X is a C ₁ -C ₁₀ alkyl group;

More preferably, X is methyl.

The preparation process of the dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant as shown in Formula I is as follows:

The specific preparation method is:

S1. Under an inert atmosphere, a 5-aminoisophthalic acid solution and an aldehyde compound solution having a structure as shown in Formula III are mixed, the temperature is raised to 5-20° C. above the boiling point of the solvent and the mixture is continuously stirred, and then the temperature is lowered to 10-50° C. below the boiling point of the solvent and the mixture is continuously stirred to obtain a mixed solution of a substance having a structure as shown in Formula II;

S2. Add DOPO to the mixed solution obtained in step S1 and react at a temperature 5-20° C. higher than the boiling point of the solvent to obtain a DOPO-based halogen-free reactive flame retardant having a terminal dicarboxyl group and a structure as shown in formula I.

The preparation method of DOPO-based halogen-free intrinsic flame-retardant nylon 66 comprises the following steps:

S1. Mixing a DOPO-based halogen-free reactive flame retardant having a terminal dicarboxyl group and a structure as shown in Formula I with hexamethylenediamine to prepare a flame retardant salt;

S2. The flame retardant salt is mixed with nylon 66 salt to carry out polymerization reaction to obtain DOPO-based halogen-free intrinsic flame retardant nylon 66.

It should be noted that the dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant used in the following embodiments is prepared by the following method:

At room temperature, add 18.12 g (0.1 mol) of 5-aminoisophthalic acid and 13.11 g (0.1 mol) of benzaldehyde into a 250 mL beaker, add 150 mL of anhydrous ethanol, and stir thoroughly to dissolve the solids;

Under nitrogen atmosphere, the above two solutions were added into a 500 mL three-necked flask, slowly heated to 85 °C, and stirred at a stirring rate of 200 rpm for 2 h;

Under a nitrogen atmosphere, the solution was cooled to 50°C and stirred at 200 rpm for 8 h.

After the reaction was completed, 21.62 g (0.1 mol) of DOPO was added to the reaction solution, and the mixture was heated to 85 °C and refluxed for 5 h with continued stirring.

After the reaction was completed, the mixture was cooled to room temperature, filtered and washed with anhydrous ethanol (150 mL×3) to obtain a white powder, which was then dried in a vacuum oven at 60° C. for 24 h to obtain 44.23 g of a dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant.

In each embodiment, the flame retardant with different R groups can be prepared by simply changing the structure of the aldehyde compound shown in Formula III.

FIG4 is a thermogravimetric analysis curve of the DOPO-based halogen-free reactive flame retardant with dicarboxyl groups and DOPO prepared by the method described in this embodiment. It can be seen that the decomposition temperature of DOPO is lower than 260° C. under N ₂ atmosphere, which cannot meet the polymerization process conditions of PA66. In contrast, the initial decomposition temperature (T _5% ) of the DOPO-based halogen-free reactive flame retardant with dicarboxyl groups prepared in this embodiment in N ₂ is 287° C. It can be seen that the DOPO-based halogen-free reactive flame retardant with dicarboxyl groups can meet the polymerization conditions of PA66.

The present invention is further described in detail below with reference to specific embodiments.

Embodiment 1

The R group of the DOPO-based halogen-free reactive flame retardant is selected as The preparation process of the dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant is as follows: the DOPO-based halogen-free reactive flame retardant and hexamethylenediamine are mixed in a molar ratio of 1:1.2, DMSO is added to dissolve, the temperature is raised to 50°C under nitrogen protection, the temperature is kept for 30 minutes and continuously stirred at a stirring speed of 50 rpm, and then the flame retardant salt is obtained after cooling and filtering.

96% of hexamethylenediamine adipate and 4% of flame retardant salt were mixed by mole, added into a 500 mL four-necked flask with a flange, heated to 260° C. under nitrogen flow, stirred at 100 rpm, and reacted for 1 hour.

The stirring speed was reduced to 50 rpm, the nitrogen flow was stopped, the vacuum was evacuated to a pressure below 100 Pa, and the temperature was raised to 265° C. and maintained for 30 min.

The heating and stirring were stopped, the nitrogen flow was resumed, the mixture was allowed to stand for 30 minutes to cool to room temperature, and the flange valve was opened to discharge the material to obtain a copolymer containing 4% by mole of the flame retardant.

The structure of the dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant used in this embodiment is as follows:

The nuclear magnetic resonance test results are shown in Figure 1, specifically:

¹ H-NMR (400MHz, TFA-d, 25℃). δ9.00-7.00 is H on the benzene ring; δ4.72 is CH on the NC bond in the flame retardant; δ4.00-1.00 is CH on the aliphatic chain.

The DSC test results of the DOPO-based halogen-free intrinsic flame-retardant nylon 66 prepared in this embodiment are shown in PA66-co-BADO ₄ in FIG. 2 . It can be seen that, like pure PA66, the nylon 66 mixed with the flame retardant is also semi-crystalline, and the mechanical properties of nylon 66 itself are retained to a certain extent.

The TG test results of the DOPO-based halogen-free intrinsic flame-retardant nylon 66 prepared in this embodiment are shown in PA66-co-BADO ₄ in Figure 3. It can be seen that compared with pure nylon 66, although the initial decomposition temperature of the DOPO-based flame-retardant nylon 66 prepared in this application is slightly lower, the residue content at 600°C reaches about 8wt%, which is significantly higher than that of pure nylon 66. This may be attributed to the phosphoric acid, polyphosphoric acid and its derivatives produced by BADO, which promote the carbonization of PA66-co-BADO _n . The flame retardancy of a polymer is directly related to its carbonization ability; high residual carbon can act as a physical barrier to prevent the material from being heated and absorbing oxygen, which shows that the DOPO-based flame-retardant nylon 66 prepared in this embodiment gives nylon 66 excellent flame retardancy.

Embodiment 2

The R group of the DOPO-based halogen-free reactive flame retardant is selected as

The DOPO-based halogen-free reactive flame retardant and hexamethylenediamine were mixed in a molar ratio of 1:1.2, DMSO was added to dissolve, the temperature was raised to 50°C under nitrogen protection, the mixture was kept warm for 30 minutes and continuously stirred at a stirring speed of 50 rpm, and then cooled and filtered to obtain a flame retardant salt.

94% of hexamethylenediamine adipate and 6% of flame retardant salt were mixed by mole, added into a 500 mL four-necked flask with a flange, heated to 260° C. under nitrogen flow, stirred at 100 rpm, and reacted for 1 hour.

The stirring speed was reduced to 50 rpm, the nitrogen flow was stopped, the vacuum was evacuated to a pressure below 100 Pa and the temperature was raised to 265° C. and maintained for 30 min.

Stop heating and stirring, resume nitrogen flow, let stand for 30 minutes to cool to room temperature, open the flange valve to discharge, and obtain a copolymer containing 6% by mole of flame retardant.

The DSC test results of the DOPO-based halogen-free intrinsic flame-retardant nylon 66 prepared in this embodiment are shown in Figure 2 for PA66-co-BADO _6. It can be seen that, like pure PA66, the nylon 66 mixed with the flame retardant is also semi-crystalline, and the mechanical properties of nylon 66 itself are retained to a certain extent; and compared with Example 1 and pure nylon 66, as the content of the flame retardant increases, the melting temperature and melting enthalpy of the copolyamide are reduced. This phenomenon shows that the addition of the flame retardant destroys the symmetry and regularity of the chain segments, restricts the movement of the chain, and leads to a decrease in the crystallization ability of the copolyamide.

The TG test results of the DOPO-based halogen-free intrinsic flame-retardant nylon 66 prepared in this embodiment are shown in PA66-co-BADO ₆ in Figure 3. It can be seen that compared with pure nylon 66, the DOPO-based flame-retardant nylon 66 prepared in this application has a slightly lower initial decomposition temperature, but the residue content at 600°C reaches 14.28wt%, which is significantly higher than that of pure nylon 66. This may be attributed to the phosphoric acid, polyphosphoric acid and its derivatives produced by BADO, which promote the carbonization of PA66-co-BADO _n . The flame retardancy of a polymer is directly related to its carbonization ability; high residual carbon can act as a physical barrier to prevent the material from being heated and absorbing oxygen, which shows that the DOPO-based flame-retardant nylon 66 prepared in this embodiment gives nylon 66 excellent flame retardancy.

Embodiment 3

The R group of the DOPO-based halogen-free reactive flame retardant is selected as

The DOPO-based halogen-free reactive flame retardant and hexamethylenediamine were mixed in a molar ratio of 1:1.2, DMSO was added to dissolve, the temperature was raised to 50°C under nitrogen protection, the mixture was kept warm for 30 minutes and continuously stirred at a stirring speed of 50 rpm, and then cooled and filtered to obtain a flame retardant salt.

90% of hexamethylenediamine adipate and 10% of flame retardant salt were mixed by mole, added into a 500 mL four-necked flask with a flange, heated to 240° C. under nitrogen flow, stirred at 100 rpm, and reacted for 2 h.

The stirring speed was reduced to 50 rpm, the nitrogen flow was stopped, the vacuum was evacuated to a pressure below 100 Pa and the temperature was raised to 250°C and maintained for 10 min.

Stop heating and stirring, resume nitrogen flow, let stand for 30 minutes to cool to room temperature, open the flange valve to discharge the material, and obtain a copolymer containing 10% by mole of flame retardant.

Embodiment 4

The R group of the DOPO-based halogen-free reactive flame retardant is selected as

The DOPO-based halogen-free reactive flame retardant and hexamethylenediamine were mixed in a molar ratio of 1:1.2, DMSO was added to dissolve, the temperature was raised to 30°C under nitrogen protection, the mixture was kept warm for 2 hours and continuously stirred at a stirring speed of 100 rpm, and then cooled and filtered to obtain a flame retardant salt.

99% of hexamethylenediamine adipate and 1% of flame retardant salt were mixed by mole, added into a 500 mL four-necked flask with a flange, heated to 260° C. under nitrogen flow, stirred at 100 rpm, and reacted for 2 h.

The stirring speed was reduced to 50 rpm, the nitrogen flow was stopped, the vacuum was evacuated to a pressure less than 100 Pa, and the temperature was raised to 270°C and maintained for 30 min.

Stop heating and stirring, resume nitrogen flow, let stand for 30 minutes to cool to room temperature, open the flange valve to discharge the material, and obtain a copolymer containing 1% by mole of flame retardant.

Embodiment 5

The R group of the DOPO-based halogen-free reactive flame retardant is selected as

The DOPO-based halogen-free reactive flame retardant and hexamethylenediamine were mixed in a molar ratio of 1:1.2, DMSO was added to dissolve, the temperature was raised to 30°C under nitrogen protection, the mixture was kept warm for 2 hours and continuously stirred at a stirring speed of 100 rpm, and then cooled and filtered to obtain a flame retardant salt.

95% of hexamethylenediamine adipate and 5% of flame retardant salt were mixed by mole, added into a 500 mL four-necked flask with a flange, heated to 250° C. under nitrogen flow, stirred at 100 rpm, and reacted for 2 h.

The stirring speed was reduced to 50 rpm, the nitrogen flow was stopped, the vacuum was evacuated to a pressure less than 100 Pa, and the temperature was raised to 260° C. and maintained for 10 min.

Stop heating and stirring, resume nitrogen flow, let stand for 30 minutes to cool to room temperature, open the flange valve to discharge, and obtain a copolymer containing 5% by mole of flame retardant.

Embodiment 6

The R group of the DOPO-based halogen-free reactive flame retardant is selected as

The DOPO-based halogen-free reactive flame retardant and hexamethylenediamine were mixed in a molar ratio of 1:1.2, DMSO was added to dissolve, the temperature was raised to 80°C under nitrogen protection, the mixture was kept warm for 60 minutes and continuously stirred at a stirring speed of 50 rpm, and then cooled and filtered to obtain a flame retardant salt.

98% of hexamethylenediamine adipate and 2% of flame retardant salt were mixed by mole, added into a 500 mL four-necked flask with a flange, heated to 260° C. under nitrogen flow, stirred at 100 rpm, and reacted for 1 hour.

The stirring speed was reduced to 50 rpm, the nitrogen flow was stopped, the vacuum was evacuated to a pressure less than 100 Pa, and the temperature was raised to 270°C and maintained for 20 min.

Stop heating and stirring, resume nitrogen flow, let stand for 30 minutes to cool to room temperature, open the flange valve to discharge the material, and obtain a copolymer containing 2% by mole of flame retardant.

Embodiment 7

The R group of the DOPO-based halogen-free reactive flame retardant is selected as

The DOPO-based halogen-free reactive flame retardant and hexamethylenediamine were mixed in a molar ratio of 1:1.2, DMSO was added to dissolve, the temperature was raised to 50°C under nitrogen protection, the mixture was kept warm for 30 minutes and continuously stirred at a stirring speed of 50 rpm, and then cooled and filtered to obtain a flame retardant salt.

99% of hexamethylenediamine adipate and 1% of flame retardant salt were mixed by mole, added into a 500 mL four-necked flask with a flange, heated to 260° C. under nitrogen flow, stirred at 100 rpm, and reacted for 2 h.

The stirring speed was reduced to 50 rpm, the nitrogen flow was stopped, the vacuum was evacuated to a pressure below 100 Pa and the temperature was raised to 270°C and maintained for 30 min.

Stop heating and stirring, resume nitrogen flow, let stand for 30 minutes to cool to room temperature, open the flange valve to discharge the material, and obtain a copolymer containing 1% by mole of flame retardant.

Embodiment 8

The R group of the DOPO-based halogen-free reactive flame retardant is selected as

The DOPO-based halogen-free reactive flame retardant and hexamethylenediamine were mixed in a molar ratio of 1:1.2, DMSO was added to dissolve, the temperature was raised to 50°C under nitrogen protection, the mixture was kept warm for 30 minutes and continuously stirred at a stirring speed of 50 rpm, and then cooled and filtered to obtain a flame retardant salt.

98% of hexamethylenediamine adipate and 2% of flame retardant salt were mixed by mole, added into a 500 mL four-necked flask with a flange, heated to 260° C. under nitrogen flow, stirred at 100 rpm, and reacted for 2 h.

The stirring speed was reduced to 50 rpm, the nitrogen flow was stopped, the vacuum was evacuated to a pressure below 100 Pa and the temperature was raised to 270°C and maintained for 30 min.

Stop heating and stirring, resume nitrogen flow, let stand for 30 minutes to cool to room temperature, open the flange valve to discharge the material, and obtain a copolymer containing 2% by mole of flame retardant.

Embodiment 9

The R group of the DOPO-based halogen-free reactive flame retardant is selected as

The DOPO-based halogen-free reactive flame retardant and hexamethylenediamine were mixed in a molar ratio of 1:1.2, DMSO was added to dissolve, the temperature was raised to 50°C under nitrogen protection, the mixture was kept warm for 30 minutes and continuously stirred at a stirring speed of 50 rpm, and then cooled and filtered to obtain a flame retardant salt.

90% of hexamethylenediamine adipate and 10% of flame retardant salt were mixed by mole, added into a 500 mL four-necked flask with a flange, heated to 250° C. under nitrogen flow, stirred at 100 rpm, and reacted for 1 hour.

The stirring speed was reduced to 50 rpm, the nitrogen flow was stopped, the vacuum was evacuated to a pressure below 100 Pa and the temperature was raised to 260° C. and maintained for 30 min.

Stop heating and stirring, resume nitrogen flow, let stand for 30 minutes to cool to room temperature, open the flange valve to discharge the material, and obtain a copolymer containing 10% by mole of flame retardant.

Embodiment 10

The R group of the DOPO-based halogen-free reactive flame retardant is selected as -CH ₂ -CH ₃

The DOPO-based halogen-free reactive flame retardant and hexamethylenediamine were mixed in a molar ratio of 1:1.2, DMSO was added to dissolve, the temperature was raised to 40°C under nitrogen protection, the temperature was kept for 2 hours and the stirring was continued at a stirring speed of 100 rpm, and then the flame retardant salt was obtained by filtering after cooling.

In terms of molar proportions, 94 mol% of hexamethylenediamine adipate and 6 mol% of flame retardant salt were mixed and added into a 500 mL four-necked flask with a flange. The mixture was heated to 245° C. under a nitrogen flow, stirred at a speed of 100 rpm, and reacted for 2 h.

The stirring speed was reduced to 50 rpm, the nitrogen flow was stopped, the vacuum was evacuated to a pressure less than 100 Pa, and the temperature was raised to 255° C. and maintained for 10 min.

Stop heating and stirring, resume nitrogen flow, let stand for 30 minutes to cool to room temperature, open the flange valve to discharge, and obtain a copolymer containing 6 mol% flame retardant.

Embodiment 11

The R group of the DOPO-based halogen-free reactive flame retardant is selected as

The DOPO-based halogen-free reactive flame retardant and hexamethylenediamine were mixed in a molar ratio of 1:1.2, DMSO was added to dissolve, the temperature was raised to 60°C under nitrogen protection, the temperature was kept for 2 hours and the stirring was continued at a stirring speed of 100 rpm, and then the flame retardant salt was obtained by filtering after cooling.

In terms of molar proportions, 94 mol% of hexamethylenediamine adipate and 6 mol% of flame retardant salt were mixed and added into a 500 mL four-necked flask with a flange. The mixture was heated to 245° C. under a nitrogen flow, stirred at a speed of 100 rpm, and reacted for 2 h.

The stirring speed was reduced to 50 rpm, the nitrogen flow was stopped, the vacuum was evacuated to a pressure less than 100 Pa, and the temperature was raised to 255° C. and maintained for 10 min.

Stop heating and stirring, resume nitrogen flow, let stand for 30 minutes to cool to room temperature, open the flange valve to discharge, and obtain a copolymer containing 6 mol% flame retardant.

Embodiment 12

The R group of the DOPO-based halogen-free reactive flame retardant is selected as

The DOPO-based halogen-free reactive flame retardant and hexamethylenediamine were mixed in a molar ratio of 1:1.2, DMSO was added to dissolve, the temperature was raised to 60°C under nitrogen protection, the temperature was kept for 2 hours and the stirring was continued at a stirring speed of 100 rpm, and then the flame retardant salt was obtained by filtering after cooling.

In terms of molar proportions, 95 mol % of hexamethylenediamine adipate and 5 mol % of flame retardant salt were mixed and added into a 500 mL four-necked flask with a flange. The mixture was heated to 250° C. under a nitrogen flow, stirred at a speed of 100 rpm, and reacted for 2 h.

The stirring speed was reduced to 50 rpm, the nitrogen flow was stopped, the vacuum was evacuated to a pressure less than 100 Pa, and the temperature was raised to 260° C. and maintained for 10 min.

Stop heating and stirring, resume nitrogen flow, let stand for 30 minutes to cool to room temperature, open the flange valve to discharge, and obtain a copolymer containing 5 mol% flame retardant.

Comparative Example 1

This comparative example only increases the molar fraction of the flame retardant salt on the basis of Example 1, as follows:

The R group of the DOPO-based halogen-free reactive flame retardant is selected as

The DOPO-based halogen-free reactive flame retardant and hexamethylenediamine were mixed in a molar ratio of 1:1.2, DMSO was added to dissolve, the temperature was raised to 50°C under nitrogen protection, the mixture was kept warm for 30 minutes and continuously stirred at a stirring speed of 50 rpm, and then cooled and filtered to obtain a flame retardant salt.

In terms of molar proportion, 85% of hexamethylenediamine adipate and 15% of flame retardant salt were mixed, added into a 500 mL four-necked flask with a flange, heated to 240° C. under nitrogen flow, stirred at 100 rpm, and reacted for 1 hour.

The stirring speed was reduced to 50 rpm, the nitrogen flow was stopped, the vacuum was evacuated to a pressure below 100 Pa and the temperature was raised to 250° C. and maintained for 30 min.

Stop heating and stirring, resume nitrogen flow, let stand for 30 minutes to cool to room temperature, open the flange valve to discharge, and obtain a copolymer containing 15% by mole of flame retardant.

Comparative Example 2

This comparative example only increases the polymerization temperature during the polymerization of nylon 66 salt and flame retardant salt on the basis of Example 1, as follows:

The R group of the DOPO-based halogen-free reactive flame retardant is selected as

The DOPO-based halogen-free reactive flame retardant and hexamethylenediamine were mixed in a molar ratio of 1:1.2, DMSO was added to dissolve, the temperature was raised to 50°C under nitrogen protection, the mixture was kept warm for 30 minutes and continuously stirred at a stirring speed of 50 rpm, and then cooled and filtered to obtain a flame retardant salt.

In terms of molar proportion, 96% of hexamethylenediamine adipate and 4% of flame retardant salt were mixed, added into a 500 mL four-necked flask with a flange, heated to 280° C. under nitrogen flow, stirred at 100 rpm, and reacted for 1 hour.

The stirring speed was reduced to 50 rpm, the nitrogen flow was stopped, the vacuum was evacuated to a pressure below 100 Pa and the temperature was raised to 300 °C and maintained for 30 min.

The heating and stirring were stopped, the nitrogen flow was resumed, the mixture was allowed to stand for 30 minutes to cool to room temperature, and the flange valve was opened to discharge the material to obtain a copolymer containing 4% by mole of the flame retardant.

Comparative Example 3

This comparative example only reduces the polymerization temperature during the polymerization of nylon 66 salt and flame retardant salt on the basis of Example 1, as follows:

The R group of the DOPO-based halogen-free reactive flame retardant is selected as

The DOPO-based halogen-free reactive flame retardant and hexamethylenediamine were mixed in a molar ratio of 1:1.2, DMSO was added to dissolve, the temperature was raised to 50°C under nitrogen protection, the mixture was kept warm for 30 minutes and continuously stirred at a stirring speed of 50 rpm, and then cooled and filtered to obtain a flame retardant salt.

In terms of molar proportion, 96% of hexamethylenediamine adipate and 4% of flame retardant salt were mixed, added into a 500 mL four-necked flask with a flange, heated to 230° C. under nitrogen flow, stirred at 100 rpm, and reacted for 1 hour.

The stirring speed was reduced to 50 rpm, the nitrogen flow was stopped, the vacuum was evacuated to a pressure below 100 Pa and the temperature was raised to 240°C and maintained for 30 min.

The heating and stirring were stopped, the nitrogen flow was resumed, the mixture was allowed to stand for 30 minutes to cool to room temperature, and the flange valve was opened to discharge the material to obtain a copolymer containing 4% by mole of the flame retardant.

Comparative Example 4

This comparative example only extends the polymerization time of nylon 66 salt and flame retardant salt in the polymerization process on the basis of Example 1, as follows:

The R group of the DOPO-based halogen-free reactive flame retardant is selected as

The DOPO-based halogen-free reactive flame retardant and hexamethylenediamine were mixed in a molar ratio of 1:1.2, DMSO was added to dissolve, the temperature was raised to 60°C under nitrogen protection, the mixture was kept warm for 30 minutes and continuously stirred at a stirring speed of 100 rpm, and then cooled and filtered to obtain a flame retardant salt.

In terms of molar proportions, 96 mol% of hexamethylenediamine adipate and 4 mol% of flame retardant salt were mixed and added into a 500 mL four-necked flask with a flange. The mixture was heated to 260° C. under a nitrogen flow, stirred at a speed of 100 rpm, and reacted for 4 hours.

The stirring speed was reduced to 50 rpm, the nitrogen flow was stopped, the vacuum was evacuated to a pressure less than 100 Pa, and the temperature was raised to 265° C. and maintained for 2 h.

The heating and stirring were stopped, the nitrogen flow was resumed, the mixture was allowed to stand for 30 minutes to cool to room temperature, and the flange valve was opened to discharge the material to obtain a copolymer containing 4 mol % of a flame retardant.

Comparative Example 5

This comparative example only shortens the polymerization time of nylon 66 salt and flame retardant salt in the polymerization process based on Example 1, as follows:

The R group of the DOPO-based halogen-free reactive flame retardant is selected as

The DOPO-based halogen-free reactive flame retardant and hexamethylenediamine were mixed in a molar ratio of 1:1.2, DMSO was added to dissolve, the temperature was raised to 60°C under nitrogen protection, the mixture was kept warm for 30 minutes and continuously stirred at a stirring speed of 100 rpm, and then cooled and filtered to obtain a flame retardant salt.

In terms of molar proportions, 96 mol% of hexamethylenediamine adipate and 4 mol% of flame retardant salt were mixed and added into a 500 mL four-necked flask with a flange. The mixture was heated to 260° C. under a nitrogen flow, stirred at a speed of 100 rpm, and reacted for 30 min.

The stirring speed was reduced to 50 rpm, the nitrogen flow was stopped, the vacuum was evacuated to a pressure less than 100 Pa, and the temperature was raised to 265° C. and maintained for 10 min.

The heating and stirring were stopped, the nitrogen flow was resumed, the mixture was allowed to stand for 30 minutes to cool to room temperature, and the flange valve was opened to discharge the material to obtain a copolymer containing 4 mol % of a flame retardant.

Experimental Example 1

This experimental example tests the mechanical properties and flame retardant properties of the nylon 66 copolymers obtained in the above embodiments and comparative examples, and the results are summarized as follows:

It can be seen from the above table that the difference between Example 1 and Comparative Example 1 is only the change in the content of the flame retardant. The molar fraction of the flame retardant in Comparative Example 1 is 15%, and the limiting oxygen index of the corresponding nylon 66 copolymer is significantly increased to 35.5%, but the tensile strength, elongation at break, melt index and room temperature impact strength of the nylon 66 copolymer are significantly reduced, resulting in the loss of the mechanical properties advantage of nylon 66 itself.

The flame retardant content of Examples 6, 7, and 8 is less than 4%, and the corresponding limiting oxygen index does not exceed 27%, and the flame retardant effect is relatively insufficient; in contrast, the flame retardant content of Examples 3 and 9 exceeds 6%, and the corresponding limiting oxygen index also exceeds 30%, but it is generally believed that a material with a limiting oxygen index higher than 27% is a flame retardant material. It does not make much sense to abandon the mechanical properties of nylon 66 itself for a higher limiting oxygen index. It can be seen that a flame retardant content of 4-6% is a more preferred range.

Moreover, the flame retardant content of Example 4 and Example 7 is 1%, and the corresponding flame retardant grade is low, but the limiting oxygen index still reaches 25%, and the surface nylon 66 still has certain flame retardant properties; therefore, it can be seen that when the flame retardant content exceeds 10% or is less than 1%, nylon 66 with excellent mechanical properties and intrinsic flame retardant properties cannot be obtained.

The above is only a preferred embodiment of the present invention, and does not limit the present invention in any form. Although the present invention has been disclosed as above in the preferred embodiment, it is not used to limit the present invention. Any technician familiar with this patent can make some changes or modifications to equivalent embodiments of equivalent changes by using the technical content suggested above without departing from the scope of the technical solution of the present invention. The implementation scheme in the above embodiment can also be further combined or replaced. However, any simple modification, equivalent change and modification made to the above embodiment based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of the solution of the present invention.

Claims (10)

1. A DOPO-based halogen-free intrinsic flame retardant nylon 66, characterized in that it is produced by copolymerizing a dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant and PA66, in which the dicarboxy-terminated DOPO-based halogen-free reactive type Flame retardants are flame retardant molecules with the structure shown in Formula I: Among them, -R is selected from C ₁ -C ₈ alkyl, phenol, furyl, aryl, heteroaryl, alkyl/alkoxy-substituted aryl, alkyl/alkoxy-substituted phenol. a kind of; Preferably, -R is selected from one of phenol, furyl, aryl, heteroaryl, alkyl/alkoxy-substituted aryl, alkyl/alkoxy-substituted phenol; More preferably, -R is selected from Where X is a C ₁ -C ₁₀ alkyl group; More preferably, X is methyl. 2. DOPO-based halogen-free intrinsic flame-retardant nylon 66 according to claim 1, characterized in that the flame retardant molecule is added by DOPO and imine bonds on the substance with the structure shown in formula II. Produced by reaction; 3. DOPO-based halogen-free intrinsic flame retardant nylon 66 according to claim 1 or 2, characterized in that the preparation method of the dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant with the structure shown in Formula I is as follows : S1. Under an inert atmosphere, mix the 5-aminoisophthalic acid solution and the aldehyde compound solution with the structure shown in Formula III, and react at a preset temperature to obtain a mixed solution of the substance with the structure shown in Formula II; S2. Add DOPO to the mixed liquid obtained in step S1 to react to obtain a DOPO-based halogen-free reactive flame retardant with a terminal dicarboxyl group having a structure shown in Formula I. 4. DOPO-based halogen-free intrinsic flame-retardant nylon 66 according to claim 3, characterized in that step S1 is specifically: Under an inert atmosphere, the 5-aminoisophthalic acid solution and the aldehyde compound solution having the structure shown in Formula III are mixed, the reaction is carried out at the first temperature, and then the temperature is lowered to the preset second temperature to continue the reaction; Preferably, in step S1, the preset first temperature is 5-20°C higher than the boiling point of the solvent; the preset second temperature is 10-50°C lower than the boiling point of the solvent; Preferably, in step S2, the reaction temperature is 5-20°C higher than the boiling point of the solvent. 5. A method for preparing DOPO-based halogen-free intrinsically flame-retardant nylon 66 according to any one of claims 1 to 4, characterized in that it includes the following steps: S1. Mix a DOPO-based halogen-free reactive flame retardant with a terminal dicarboxyl group as shown in Formula I and hexamethylenediamine to prepare a flame retardant salt; S2. Mix the flame retardant salt and nylon 66 salt for polymerization reaction to obtain DOPO-based halogen-free intrinsic flame retardant nylon 66. 6. The preparation method of DOPO-based halogen-free intrinsic flame retardant nylon 66 according to claim 5, characterized in that, in step S1, the DOPO-based halogen-free reactive flame retardant with dicarboxylic terminal group and hexamethylene diamine are mixed with 1 (1-1.5) is mixed in a proportion and then a solvent is added, the temperature is raised and stirred for a preset time to form a mixed liquid, and the mixed liquid is post-processed to obtain a flame retardant salt; Preferably, the molar ratio of the dicarboxyl-terminated DOPO-based halogen-free reactive flame retardant to hexamethylenediamine in the mixed solution is 1:1.2. 7. The preparation method of DOPO-based halogen-free intrinsic flame-retardant nylon 66 according to claim 5 or 6, characterized in that in step S1, the dicarboxylic DOPO-based halogen-free reactive flame retardant and hexamethylene diamine are DMSO is used as the solvent, and the flame retardant salt is produced by the mixing reaction; the mixing temperature is 30-100°C; Preferably, the preset time is 0.5-2h; More preferably, the stirring speed is 50-100 rpm. 8. The preparation method of DOPO-based halogen-free intrinsic flame-retardant nylon 66 according to claim 5, characterized in that the post-treatment is: cooling the mixed solution to room temperature and filtering under reduced pressure, washing and drying Finally, the flame retardant salt is obtained; Preferably, the washing and drying include: washing with absolute ethanol, drying at 60°C for 24 hours, then washing with deionized water, and drying at 95°C for 24 hours; More preferably, the specific operations of washing and drying are: wash three times with absolute ethanol, put it in a vacuum oven, and dry it at 60°C for 24 hours; then wash it three times with deionized water, and put it in a vacuum oven after washing with water. Dry at 95°C for 24 hours. 9. The preparation method of DOPO-based halogen-free intrinsic flame retardant nylon 66 according to claim 5, characterized in that, in step S2, the molar ratio of nylon 66 salt and flame retardant salt is (90-99): ( 10-1); Preferably, the molar ratio of nylon 66 salt and flame retardant salt is (94-96): (6-4). 10. The preparation method of DOPO-based halogen-free intrinsic flame-retardant nylon 66 according to any one of claims 5-9, characterized in that the polymerization process is as follows: mix the flame retardant salt and nylon 66 salt, and heat it to a predetermined temperature. Carry out the reaction at the preset third temperature, then reduce the pressure in the reactor to below 100Pa, raise the temperature to the preset fourth temperature to continue the reaction, stop heating and stirring after the reaction is completed, and obtain DOPO-based halogen-free intrinsically flame-retardant nylon 66; The preset third temperature is lower than the preset fourth temperature; Preferably, the preset third temperature is 240-260°C; Preferably, the reaction time at the preset third temperature is 1-2h; Preferably, the preset fourth temperature is 250-270°C; Preferably, the reaction time at the preset fourth temperature is 10-30 minutes.